1. What does the term "tidal volume" refer to? Are there other breathing volumes that are measured?

Just as we think of the tide moving in & out on the seashore, the term "tidal volume" refers to the amount of air moving in & out of our lungs during 1 normal inhalation & exhalation.

There are 3 more important respiratory volumes that reveal the health & capacity of the lungs:

inspiratory reserve volume: the amount of air that can be forcefully inhaled into the lungs after a normal inhalation.

expiratory reserve volume: the amount of air that can be forcefully exhaled after a normal exhalation.

residual volume: the air that is still remaining in the lungs after a forced exhalation.

By adding together 2 or more of these volumes, various "capacities" of an individual's lungs can be determined. For example, a person's total lung capacity equals his/her inspiratory reserve volume, plus the tidal volume, plus the expiratory volume, plus the residual volume.

Figure 16.15 and Table 16.2 chart & define these terms.
 
 

2. Oxygen is transported from the lungs to tissues by the hemoglobin molecule in red blood cells. Is that the way carbon dioxide is transported also?

Most CO2 (about 70%) is carried in the blood in the form of the bicarbonate ion, HCO3-. The carbon dioxide is a normal waste product of cell activity. As its levels build up in tissues, it diffuses to locations where there is less carbon dioxide, such as into blood. There it can diffuse briefly into red blood cells, but it quickly reacts with water in the cell to form carbonic acid (H2CO3). The acid dissociates into hydrogen ions (H+) & carbonate ions which diffuse out of the red blood cell into the blood plasma. For this reason, although some CO2 is traveling temporarily as a dissolved gas in the plasma and some of it is traveling in the red blood cell, the larger percentage of CO2 has been processed quickly into the carbonate ion by the chemical reaction inside the red blood cell and will diffuse back out into the plasma.

Figure 16.22 illustrates chemical changes.
 
 

3. What is the "respiratory membrane" that oxygen must diffuse through to reach the blood stream?

Oxygen will move by passive diffusion from inside the space of an alveolus through the layer of simple squamous epithelium that forms the alveolus & its "basement membrane" (not a true membrane, but an area containing elastic & collagenous fibers), across the space between the alveolus & the capillary surrounding it, then through the basement membrane of the capillary & the simple squamous tissue of the capillary itself (endothelium).

Anything that causes thickening of these membranes, such as emphysema or lung cancer, will make breathing more difficult.

The diffusion of carbon dioxide OUT of the capillary & into the alveolus for exhalation will follow this same pathway in reverse.

Figure 16.19 illustrates the relationship of the cells of an an alveolus and a capillary.

(Diffusion distance is also increased by the surfactant that sits on the inside of the alveolus. Conditions such as pneumonia, asthma or cystic fibrosis which add mucus to the respiratory passageways will complicate the diffusion of oxygen & carbon dioxide.
 
 

4. Why does oxygen move from the inside of an alveolus into a capillary?

As you learned in the chapter on chemistry & atoms, molecules tend to move from an area where they are in higher concentration to an area where atoms of that particular substance are in lower concentration. This is called diffusion. Presumably, when a person inhales, there is plenty of oxygen in the inhaled air. The tendency for oxygen to move across the respiratory membrane is increased by the fact that the partial pressure of oxygen (PO2) in the alveolus is greater than the partial pressure of oxygen in blood returning to the lungs from the body. The reverse situation is true for carbon dioxide: the partial pressure of carbon dioxide (PCO2) in blood returning to the lungs is greater than the partial pressure of carbon dioxide in the alveolar air, so carbon dioxide will diffuse to the region where where it is present at a lower partial pressure.